This invention involves an improved method and apparatus of multiplexing optical data. In particular, the present invention pertains to a method and apparatus for polarization division multiplexing data which is to be transmitted in an optical data transmission or communication system.
The communications and information service industries have been able to vastly improve existing services and branch into new services as a result of advancing data transmission and communications technologies. The field of optical data transmission is one such technological area in which a number of advances has been made. Optical communication and data transmission systems have recently been an especially fast growing segment of the information infrastructure. Tens of thousands of miles of fiber optic cable have been installed to create optical networks interconnecting the globe. These optical networks enable tremendous amounts of data to be transmitted.
An early use of optical networks was for the transmission of voice signals in telecommunication applications. At present, a number of forms of data and information are being transmitted via optical networks. For instance, optical networks are commonly found in such applications as cable television, video-on-demand, interactive services data transmission, multimedia signals, fields employing optical instruments, as well as many other technologies. Data transmission via optical networks is advantageous because it has low delay and offers large bandwidth.
A disadvantage of optical networks is the relatively high cost associated with the installation of these networks. Since the high cost of optical data transmission must be passed on to subscribers or users of services, cost is a drawback to service providers that consider implementing optical networks. Even though optical data transmission is widely used in a number of applications, the burdensome costs (e.g., costs per line) of installing fiber-optic cable effectively impedes the expansion of optical networks into lower margin functions or services. One way of reducing costs is to increase efficiency in data transmission. That is, if more data can be transmitted using the same amount of bandwidth (e.g., within the same fiber), the cost per line of optical data transmission can be lowered.
One conventional approach to increasing the data rate of optical data transmission is through the use of wavelength division multiplexing (WDM), in which multiple different wavelengths are carried over a common fiber-optic waveguide. WDM separates a given, or allocated, bandwidth into several channels and provides a more efficient use of the available bandwidth than transmitting over fiber-optic cables without wavelength channelization. Dense channel wavelength division multiplexing (DWDM) is an offshoot of WDM through which the bandwidth may be separated into 32 or more channels. Through use of techniques such as WDM or DWDM, fiber-optical telecommunications systems may be more efficiently utilized.
Specialized equipment is needed to combine and then separate optical signals in wavelength division multiplexed systems. In practice, a number of devices have been used for optical multiplexing in WDM fiber-optic telecommunications systems, as well as in the field of spectroscopic analysis. Such devices include, for example, optical diffraction gratings, prisms, and various optical filters. The conventional devices required to implement WDM tend to be expensive, and require light sources with a relatively narrow optical spectrum, or filters with a relatively narrow bandwidth, in order to multiplex the signal per wavelength. In addition, such conventional devices are adversely affected by environmental fluctuations such as conditions of varying temperatures.
Accordingly, it would be desirable to develop new techniques for efficiently transmitting information over optical media that overcome the problems associated with WDM devices.
The present invention is drawn to an apparatus and method for polarization division multiplexing.
Generally speaking, exemplary embodiments are directed to an optical polarization multiplexing system having two orthogonal polarization filters at the transmitting end, and two corresponding polarization filters at the receiving end. A first optical signal to be transmitted is subjected to one of the two polarization filters at the transmitting end. A second optical signal to be transmitted, having the same frequency as the first optical signal, is subjected to the other polarization filter. The two polarized signals are then added by an optical adder to produce a single (composite) optical transmission signal having two variously polarized components. The composite optical transmission signal is transmitted via an optical link to an optical splitter at the receiving end, which splits the signal into two signals. Each of these signals is routed towards an optical polarization filter. One of the polarization filters recovers the first optical signal, and the other polarization filter recovers the second optical signal.
The present invention overcomes the drawbacks of conventional systems set forth above and other shortcomings of conventional optical multiplexing systems. For instance, one advantage of the polarization division multiplexing system according to the present invention is the more efficient use of spectrum to communicate data. That is, more data can be conveyed using the same amount of optical bandwidth than was possible using conventional approaches. Another advantage of the present invention is that through use of polarization division multiplexers, optical receivers and transmitters having relatively wide optical spectrums may be employed, as compared to conventional optical multiplexers. Another advantage of the present invention is the decreased sensitivity with respect to the wavelength of optical transmitters and receivers to environments subject to varying temperature conditions. Another advantage of the present invention, is that polarization division multiplexing may be used in conjunction with conventional multiplexing or optical bandwidth optimizing systems to provide increased data transmission efficiency.